THE DESERTS OF NORTH AMERICA The North American deserts are highly accessible, well-researched and very diverse, so they provide a good basis for understanding desert ecology. These deserts are found in a broad band running down the western side of the USA and into Mexico. They lie in a large basin between the Rocky Mountains to the east and the Sierra Nevada to the west. They receive relatively little water - typically less than 25 cm (10 inches) per year - because most of the precipitation falls on the higher mountain ranges and not in the lower-lying desert regions (which are in the "rain shadow" of the mountains). Approximate boundaries of the four main deserts of North America These North American deserts are grouped into four major types - the Great Basin Desert, Sonoran Desert, Chihuahuan Desert and Mojave Desert - depending on their characteristic physical features (rainfall, topography, soil types) and characteristic vegetation and associated animal communities. The Great Basin Desert is the largest desert area of North America. It is also the most northerly, covering most of Nevada (Ne), the western third of Utah (U) and parts of Idaho (Id) and Oregon (Or).

It is a cold desert because of its northerly location and its relatively high altitude - most of the land lies above 1200 metres (4000 ft), but in the 'rain shadow' of the higher mountain ranges. Much of the precipitation occurs as winter snowfall, but not all of this melts into the ground because some of it evaporates in spring. The vegetation tends to be very uniform over large areas of this desert. It is dominated by various types of sagebrush , or by saltbush where the soil has a high salt concentration. In fact, the soils often have a high salt content (sodium and calcium ions) caused by evaporation of water in the hot summer months, and no vegetation can grow in the saltiest regions. The Mojave Desert occurs further south and covers the southern part of Nevada and part of California (Ca) but elements of it extend into Arizona (Az) where it blends into the Sonoran Desert. Again, it is classed as a cold desert because of the low winter temperatures. The precipitation occurs in winter, usually as rain but sometimes as snow at the higher elevations.

The features of the Mohave Desert are difficult to define because of the marked variation in topography, soils and climate. For example, the northern section is composed of low-growing shrubs, similar to those of the Great Basin Desert, whereas the southern section blends into the Sonoran Desert, with extensive tracts of creosote bush. The Mohave Desert includes Death Valley - the lowest (below sea level) and driest of all desert regions, where there may be no rain for several years. The Majove Desert contains some highly characteristic plants and animals - most notably the joshua tree at higher elevations. The Sonoran Desert covers the southern part of Arizona and part of California, but extends south into the mainland of Mexico and into the extended isthmus of the state of Baja California (BC) in Mexico. The Sonoran Desert is a hot desert and, unlike all the other desert regions of North America, it receives both winter and summer rains. This pattern of rainfall is caused by the seasonal shifts of major storm tracks across the USA. The Sonoran Desert receives winter rainfall from moisture-laden air carried on winds from the Pacific Ocean, and summer rainfall from air carried northwestwards from the Gulf of Mexico.

As a consequence, parts of the Sonoran Desert can support unusually lush vegetation, including several trees and sub-trees, and some very large cacti such as the saguaro and cardon . The Baja California peninsula of Mexico is also included in the Sonoran Desert, but the west-facing slopes of this peninsula receive moisture-laden air from the Pacific Ocean and have some uniquely lush vegetation, including epiphytic plants that gain their moisture from the sea mists. The Chihuahuan Desert occupies the extreme west of Texas (Tx) and part of New Mexico (NM), but the largest part of this desert occurs in mainland Mexico. This desert region receives summer rains from the Gulf of Mexico - typically about 20-30 cm per year. Over much of this desert the soils are derived from calcareous rocks and thus have relatively high pH. The Chihuahuan Desert also lies at relatively high elevation (typically about 1200 metres, or 4000 ft) and thus has cool winters with periodic frosts, but the summers are hot. The combination of relatively high rainfall, calcareous soils and cool winter temperatures favours the growth of grasses, yuccas and agaves .

There are many small cacti , but few of the larger cacti associated with the Sonoran Desert. Whilst some of the best desert areas of North America are now protected from development by being designated as State Parks, National Parks or National Monuments, vast areas of land dominated by creosote bush and other less spectacular vegetation are now been used for irrigated agriculture, drawing water from major rivers. When irrigated, these desert can be phenomenally productive because of their year-round warmth and solar intensity. Irrigated agriculture (right) compared with the natural vegetation (left and foreground) North American Desert | region, North America | Britannica.com North American Desert Mojave Desert North American Desert, vast, irregular belt of inhospitable terrain that stretches north to south down the western side of the North American continent from southern Oregon and Idaho to northern Mexico . It roughly corresponds to the sheltered, and hence rain-starved, intermontane region lying between the soaring barrier of the Rocky Mountains to the east and the moist, fertile coast ranges fringing the Pacific Ocean to the west.

Estimates of the size of the region vary, depending upon definition and year, from about 500,000 to more than 730,000 square miles (1,300,000 to 1,900,000 square kilometres). Physical features of western North America. Encyclopædia Britannica, Inc. The physical geography and human utilization of this huge area exhibit great internal variety, but its overall aridity—associated with an excess of potential evaporation over precipitation, great temperature extremes, frequent winds, localized storms, and a predominance of starkly eroded, sun-beaten landscapes—give it an unquestioned unity. Scientists, in naming this whole ecological complex, or biome, the North American Desert, are merely echoing the legend of a “Great American Desert” established as early as the 1820s by the vivid reportage of an expedition led by the American pioneer explorer and engineer Stephen H. Long. All forms of life, from lowly plants and insects to people, have had to struggle to survive in the region, and the North American Desert thus has had enormous importance in the development of the continent.

Descendants of the earliest inhabitants, the desert-culture Indians, are still found in the area. Their ranks were swollen in the 19th century by tribes thrust westward in the great dispossession that followed the advance of European settlement from the Eastern Seaboard into the continental interior, and the region is now the home of most of the U.S. Indian population. The legacy of much earlier population movements from farther south has lent a distinctly Spanish element to the area, while modern American settlement has added its own contribution in the form of sheep and cattle grazing, military installations, and small but often rapidly expanding oases of mining and manufacturing. All the peoples of the North American Desert, whatever their origin, have their lives molded by the basic and all-pervading lack of water and minimal vegetation cover. Physical features Sahara Physiography Differences in latitude, elevation, climate, topography , vegetation, soil, and human use have allowed the North American Desert to be subdivided into regions of cold mid-latitude and hot mid-latitude desert.

The continent’s major deserts are associated with the intermontane Basin and Range Province of the western United States , northern Mexico, and the Colorado Plateau region of northern Arizona, southwestern Colorado, northwestern New Mexico , and southeastern Utah. Classified as a cold mid-latitude desert is the Great Basin Desert , which lies in the physiographic Great Basin that covers nearly all of Nevada, western Utah, and portions of surrounding states. It constitutes the largest section of the Basin and Range Province and includes the Great Sandy Desert of southeastern Oregon, the Snake River Plains of southern Idaho, the volcanic Black Rock Desert of northwestern Nevada, and the Great Salt Lake Desert of western Utah. Although they are not usually assigned to the physiographic Great Basin, the Red Desert of southern Wyoming and the Painted Desert of northern Arizona are sometimes considered as extensions of the Great Basin Desert. Painted Desert, north-central Arizona. Jon Sullivan To the south the Great Basin Desert merges with the Mojave (Mohave) Desert and the hotter mid-latitude deserts.

In California near the undefined Great Basin–Mojave border are the sun-scorched landscapes of Death Valley; the valley’s maximum depth of 282 feet (86 metres) below sea level is the continent’s lowest point. The Mojave merges in the southeast with the Sonoran Desert , which covers much of Baja California and runs along the Gulf of California coast to the Mexican state of Sonora. Its subdivisions include the Yuma and Colorado deserts. To the east lies the huge Chihuahuan Desert , extending from southern New Mexico (where the dazzling gypsum dunes of White Sands National Monument are found) and western Texas to the highlands of central Mexico. Death Valley National Park in the Great Basin, southeastern California, U.S. Joseph Sohm—Choromosohm/Photo Researchers Ringling Bros.

Folds Its Tent Most of the North American Desert—and all the salty-lake remnants—occupies areas covered by geologically recent Quaternary deposits (i.e., those less than 2.6 million years old) and mountains thrust up and folded by movements in the Paleogene and Neogene periods (about 65 to 2.6 million years ago), with some plains and plateaus consisting of sediments up to 225 million years old. USA Facts Through long periods of erosion, the landforms produced from these rocks have become characteristically sharp and angular (except in the heavily eroded badlands regions) and contain, in their harsh but breathtaking beauty, some of North America’s finest scenery. The action of wind, temperature changes, ephemeral streams, and floods, coupled with the general lack of dense vegetation and deep soils, have all been involved in this molding process. The individual deserts are characterized by plateaus, gorges, ravines, and alluvial fans washed out at the feet of mountains. Deserts of the bolson type contain playas (dried-up lake remnants) and mud and salt lakes and flats.

Deserts of the hammada type are characterized by extensive rocky surfaces with boulder or gravel coverings, sometimes blackened and wind-scoured, with magnificent buttes, mesas, and other isolated mountain remnants rising high above the flat landscape. Stretches of shifting sands known as ergs —the extensive Algodones Dunes of the Colorado-Yuma desert are a notable example—are found at lower elevations, with the shallow troughs of arroyos carrying intermittent streams from surrounding uplands to be lost in the sands. Soils The soils of the North American Desert have origins similar to those of more humid regions, but they are less enriched by organisms and less leached of constituents . On older surfaces most belong to the dry-soil order called aridisols, but local variations occur, reflecting differing salt and mineral composition and presence or absence of organic matter. On younger eroded surfaces or those infilled with alluvial (riverine) sediment, weakly developed entisols occur. With proper management, the more fertile soils can be productive.

Climate Facebook Twitter YouTube Instagram Pinterest Regardless of the latitude, longitude, and elevation, a harsh and extreme temperature regime exists in the North American Desert, with a high frequency of cloudless days and intense solar radiation, infrequent and intense storms, winds carrying dust and sand, low relative humidity, and rapid evaporation or sublimation of snow. Regionally specific climatic zones exist, ranging from temperate in the Great Basin and Painted Desert to subtropical in the western Sonoran and northern Chihuahuan sections to tropical in the Sonoran gulf coast. Average annual temperatures range from approximately 49° F (9° C) in the northern deserts (Black Rock) to 73° F (23° C) in the Sonoran Desert and 77° F (25° C) at the bottom of Death Valley .

Average annual rainfall varies from 6 to 12 inches (150 to 300 millimetres) in the Great Basin Desert, 2 to 6 inches in the Mojave Desert (a low of 2 inches in Death Valley), 10 to 11 inches in the Red and Painted deserts, 8 to 18 inches in the Sonoran Desert, and 12 to 14 inches in parts of the Chihuahuan Desert. Precipitation may be seasonal and characteristically is erratic, scattered, or intense. Snowfall of 12 to 24 inches occurs in most of the northern deserts, decreasing to only a trace in the Sonoran Desert. Winds are directional, with speeds averaging seven to nine miles per hour. Average relative humidities are generally less than 50 percent (20 percent for Death Valley), but 10 percent or less is not uncommon. The growing season varies from 90 to 140 days in the northern deserts to more than 300 days along the Sonoran gulf coast and in the Baja Peninsula. Plant life In the North American Desert, available moisture is the most critical factor for life.

Local environmental factors also are significant in determining the nature of desert plant communities and their dependent animal life. Most desert plants are xerophytes (plants adapted to arid conditions) or phreatophytes (deep-rooted plants that are dependent on a permanent water supply and survive by tapping groundwater). Sagebrush and saltbush characterize the Great Basin region, with Joshua trees, creosote bush, and burroweed typical of the Mojave. The Sonoran Desert has a thorn scrub of shrubs (mesquite, paloverde, ironwood, burrobush, smoke tree, and cat’s claw) and a great variety of moisture-preserving succulents, such as cacti. The Arizona Upland Desert is noted for the giant saguaro and barrel cacti, while the Chihuahuan Desert is characterized, notably in its eastern part, by a ground cover of open mesquite, a scattering of larger trees, and shrubby undergrowth, including the yucca, prickly pear, agave, and other varieties of cactus. Plant life and associated algae, lichens, mosses, and insects become more complex as temperature and moisture conditions improve.

Survival strategies of ephemeral and perennial plants found in the deserts of Arizona. Encyclopædia Britannica, Inc. Sonoran Desert, U.S. Deserts, North America Deserts - National Geographic Written by Douglas H. Chadwick Republished from the pages of National Geographic Magazine, September 2006 A desert can fool the eye. A sun-blasted plain of death turns suddenly into a landscape of sound, water, and life. If the Sonoran Desert's a wasteland, why is the vegetation so thick that it's nearly impossible to see over, much less walk through unpoked? If life here has been sun-blasted to a minimum, why are the sandy washes scribbled with footprints from javelinas, mule deer, ringtails, and so many rodents that as many as 200 rattlers a square mile can make their living with a sit-and-wait hunting technique? This seems to be an ecosystem in need of a public relations makeover. Yes, the Sonoran Desert does get parched and hot as hell.

But its emblem, that saguaro standing with upraised arms, is not some lonely perch for vultures in the barrens but part of a forest of storage tanks juicy with life. Within hours of a rain, many will be spreading new rootlets to harvest the drops. The plants' accordion-like structure lets them swell with extra liquid, as a Gila monster's extra-large bladder is designed to do, while it packs away food reserves in its expandable tail. During May and June, the driest months, when winter's rains have been all but forgotten by most, the saguaro and its even bigger southern counterpart, the cardon cactus, crown themselves with extravagant, white, nectar-filled blossoms. These nourish birds, insects, and especially bats, and are pollinated in return. The flowers then develop into succulent fruits, supplying meals and moisture to a still broader array of creatures, from iguanas to kit foxes, until the summer thundershowers begin.

Retiring to one of the many small trees that also characterize this desert, such as yellow paloverde, blue paloverde, catclaw acacia, ironwood, or honey mesquite, to rest and digest, the animals leave droppings full of seeds exactly where a saguaro or cardon needs to grow during its tender years: in the shade of a nurse plant. If life here is just hanging on by a thread, how is it that a saguaro may produce millions of seeds a year and live to be 250 years old? With few winter frosts and dual rainy seasons half a year apart, the Sonoran region supports such a rich variety of flora and fauna that it seems almost lush compared with other deserts. In some experts' view, it isn't really a desert so much as a drier version of the subtropical thornscrub found farther south in Mexico. However you define the Sonoran ecosystem, it is a spectacular illustration of how communities of organisms facing extreme conditions find ways not merely to endure but to flourish. A good place to rest, this fold of shade at the arroyo's steep edge, and a good place to ponder.

Two days ago, a July cloud-burst dropped an inch of rain here in Arizona's Saguaro National Park. Water is still seeping down the arroyo and collecting in pools. Bright green algae already coat the bottoms. Mourning doves, cactus wrens, and hundreds of bees sip from the edges. And somehow, among these mountainsides of stones too hot to touch, tadpoles have materialized within the pools. Native Americans who paused beneath the same rock wall long ago left paintings of humans, creatures, and spirits. If, as it seems, the figures are keeping watch, they have witnessed many times before the drama of survival that is about to play out. Many frogs and toads need months to develop from eggs into adult form. Spadefoots, which are similar to true toads but have smooth skin, catlike eyes, teeth, and a sharp nail on the hind feet, live by different rules. Adults exist in a dormant condition buried beneath the desert floor like plant bulbs until they sense vibrations from thunder and raindrops. Then they emerge to gather at runoff pools, send ear-splitting choruses into the night, and breed.

With metabolisms set on fast-forward, the offspring can transform into miniature grown-ups in as few as eight days. The race is on to become an air-breather hopping about and fattening on insects before the pond dries, leaving the moist-skinned amphibians no choice except to spade back underground and wait for another heavy rain. That might be months. It might be more than a year. If the tadpoles in the arroyo pools are red-spotted toads, usually found around more permanent water, they're doomed. If they're spade foots, they have a chance. Every day, they grow a little larger. Everyday, the pools are smaller, vaporizing under the midday sun…. "A year ago this month, she was on death's door. Skin draped over a skeleton. Out moving all the time, got to find food. But there was none," says Jon Davis, a graduate student from Arizona State University. "I wanted to take her in, I was so sure she would die." In the light of our headlamps, a 15-inch (38-centimeter) Gila monster, firmly held behind the jaw, dangles like a toy dragon covered with pink and black beadwork.

The belly is wide, the wriggling tail plump as Polish sausage. We can see baby rabbit fur stuck to the mouth. Life for this reptile is plainly on the upswing. Davis pulls a couple of cactus spines from the female's toes and puts her in a bag for closer inspection by daylight. Rising with the calls of doves and quail before dawn, he reaches in to grab the animal and scan her with a portable ultrasound unit. A wake-up cup of coffee first is a good idea; the Gila and its closest kin, the Mexican beaded lizard, are both venomous, and their bites can cause severe pain, faintness, or worse. An image of the animal's bladder appears on the screen. This organ has banked a fortune in water, the currency of the Sonoran Desert ecosystem. "Gilas can drink enough in one day to boost their body weight 20 percent," he tells me. "Just like that, they'll go from looking awful to looking great.

This is the kind of year that gets everybody through." Spanning 100,000 square miles (260,000 square kilometers), the Sonoran Desert encompasses southeasternmost California, southwestern Arizona, at least half the Mexican state of Sonora, and nearly all of the Baja California peninsula. Although some sections average barely 3 inches (8 centimeters) of rain yearly, others see 10 to 12, more than parts of Wyoming. Winter storm systems from the Pacific typically supply about half the annual total, but the winter of 2004-05 brought record deluges, breaking the grip of a several-year drought. By spring, the region looked and smelted like a florists' convention. Even plants no one had seen for decades erupted from long-patient seeds. Then summer came. The thermometer hit 100 degrees Fahrenheit (37.8 degrees Celsius) or more for 39 straight days in Tucson. Phoenix recorded afternoons of 115 degrees Fahrenheit (46.1 degrees Celsius) and more than two dozen heat-related deaths.

Heatstroke and dehydration were killing at least that many illegal immigrants from Mexico monthly as they trekked north through remote Arizona borderlands. Lowest and warmest of North America's four major deserts (the others being the Mojave, Great Basin, and Chihuahuan), the Sonoran is also the only one with two distinct rainy seasons. That tremendous summer heat rising off the countryside draws moist air from the Gulf of California and occasionally the Gulf of Mexico. Thunderclouds take over the blazing sky many afternoons, delivering intense downpours, complete with flash floods. The monsoons, as locals call the hot season storms, were late this year. But they have arrived, slaking thirsts and spurring another round of plant growth. Which means more births among rabbits, rodents, and birds, whose young are all favorite meals of the Gila monster. It may eat enough at one nest to increase its weight as much as 50 percent, then retire to its burrow for a week or more. A hormone recently discovered in the monsters' venomous saliva may help regulate this on and off pattern of activity.

In 2005 a synthetic version of the chemical won approval as a drug that has proved very effective in controlling type 2 diabetes. It could aid patients in losing weight at the same time. Davis has spent three years tracking Gilas implanted with radio transmitters and miniature temperature recorders. His study site near Picacho Peak State Park, northwest of Tucson, samples a single square mile of Sonoran Desert. It contains at least two dozen Gilas. In an adjoining square mile, another reptile researcher estimates the western diamondback rattlesnake population at 200. That's not counting the tiger rattlers, Mojave rattlers, and sidewinders. A lot of Sonoran wildlife shifts to nighttime activity in summer to cope with the heat, and while tracking Gilas under the stars, we often come upon diamondbacks. Each rests in a motionless coil, scanning the darkness with infrared sensors in pits on its face, until a kangaroo rat or pocket mouse happens by. Scorpions and tarantulas scuttle underfoot as we ease past cactuses furry with long spines. One careless moment, and you jump back with pieces pinned to your hide.

Meet teddy bear cholla, possibly the worst-named, least cuddly plant in the world. One by one, the pools vanish. Shriveled tadpole bodies cover the pebbles like tar. Where a few puddles no bigger than a dinner plate persist near the source of the seep, so do squirming masses of young amphibians, their bodies still growing, backs breaking the surface at times. Although great, luminous clouds billow overhead most afternoons, the showers they yield are spotty. Finally, it does rain on the mountainside. Not much; just enough to swell the pools a fraction.... Even in early May, this desert can draw three gallons of water out through your skin in a day. But if the region is so arid, why are there angelfish swimming with me and curious sea lions swooping by? Half of the Sonoran ecosystem lies within 50 miles (80 kilometers) of a sea. Salt water surrounds one of the most fascinating portions, which takes in nearly all of the 34 major islands and more than 850 smaller islands and islets in the Gulf of California.

Some have labeled this archipelago Galápagos North because so many different species arose from common ancestors as they adapted to island environments, providing a laboratory for the study of evolution. Steep-sided lsla San Pedro Mártir, a square mile in size, towers among the remote Midriff Islands at the center of the gulf, shimmering ghostly white from guano, blurred by a haze of wings, and moated by cold, upwelling currents where great whales feed on krill and sardines. Large birds, including the world's densest colony of brown boobies and largest colony of blue-footed boobies, rule this strange castle alongside side-blotched lizards, which abound almost a thousand to an acre. The court jesters are striped gnats known as bobitos, or little boobies. While gnat larvae develop in the boobies' droppings, adults sip moist mucus from the birds' eyelids. They're happy to drink around your eyes, too. And in your ears, up your nose, or on any patch of sweaty skin—by the hundreds, tickling without letup.

The green-and-turquoise lizards, Uta palmeri, found only on San Pedro Mártir, are almost as bold. Show a bit of red on your sock, and a dozen emerge from rocks to run up your leg and bite the cloth. Red happens to be the color of a key Uta food: fruit from cardon cactuses, which grow in candelabra forests fertilized by the bird lime. The lizards also dash right in among the birds to snatch fish scraps and gnats. There are 115 kinds of land-dwelling reptiles on the gulf's islands. Forty-eight of them are unique to this region. For example, the Midriff island called San Esteban has produced a rattlesnake, whipsnake, spiny iguana, and Gila monster-size chuckwalla, all found only within its 16 square miles (41 square kilometers). Ana Luisa Figueroa of the Mexican resource agency CONANP (Comisión Nacional de Áreas Naturales Protegidas) urged me to add this: By all means, go see the wonders of the gulf's isles.

But do it with a guide or, better yet, from a boat circling the shores. The birds nest so close together that a single hiker can put thousands to flight. Before that blizzard settles down, every exposed egg or young chick will have been eaten by gulls or ravens. Many of the gulf islands have native Peromyscus mice. Many host the endemic Mexican fishing bat. But hardly any hold larger predatory native mammals. After seeing me off on my snorkel along San Pedro Mártir's shore, Araceli Samaniego, a biologist with the Mexican nonprofit Grupo de Ecología y Conservación de Islas, part of a North American island conservation network, scrambled up steep cliffs where she noticed a dwindling colony of red-billed tropicbirds nesting. She returned fuming, carrying crab shells and infant bird bones, all gnawed. "Rats," she grumbled. "Everywhere!" House mice—Mus musculus—and cats, too, often jump ship to take up island life. At each island we visit, Samaniego sets out a series of baited live traps.

A petite, soft-spoken woman of 28 with an easy laugh, she talks to the wild mice she catches, calling them corazón—sweetheart—and stroking their little feet before letting them go. If the traps hold rats instead, she carefully weighs and measures them. Then she kills them. She doesn't like it, but she likes the idea of rats gobbling nestlings and rare endemic life-forms even less. Two-thirds of all known extinctions worldwide since the year 1600 have taken place on islands, Samaniego reminds me, and alien species introduced by people are a major cause. Gathering data by trapping is the early stage of a planned effort by the island conservation group and CONANP to eradicate rats on San Pedro Mártir. A similar program on nearby IsIa Raza in 1995 helped restore the globe's premier breeding colonies of elegant terns and Heermann's gulls, though it came too late for the island's nesting black storm-petrels and Xantus's murrelets. The arroyo has exactly three pools with huddled tadpoles left.

Another shower swept the mountainside last evening; too brief to expand the pools, it at least replenished the trickle that sustains them. Other life-forms are growing and changing on all sides. Having turned entire slopes yellow with blossoms in spring, then withered into lifeless-looking bundles of twigs, the brittlebush is going green again. Ocotillos on the banks sport new leaves. Wildflowers tint the shade, conducting business not just with honeybees but also with burly carpenter bees that drill into woody plant stalks to build brood chambers or their larvae, and with tiny, emerald, stingless bees that live a solitary existence. An entomologist in Tucson, Steve Buchmann, thinks the Sonoran ecosystem may support the greatest diversity of bees anywhere—around 500 species. Why not? Plants of one kind or another are in bloom almost every day of the year. As temperatures soar dangerously high, honeybees literally keep a cool head by extruding a water droplet from their mouth to shed heat by evaporation. The afternoon is hotter by the hour.

Yet even as some of the tadpoles trapped in cracks thrash half exposed to the air, new ranks of thunderclouds are forming not far to the south…. In 1993, 2,760 square miles (7,150 square kilometers) of Mexican mainland near the head of the gulf were declared the Pinacate and Grand Altar Desert Biosphere Reserve. The Desierto de Altar segment holds North America's most extensive dunes—finally, a setting that matches the Sonoran Desert stereotype. The sands are shavings off landscapes like the Grand Canyon and the snowy peaks of the Rockies, swept downstream by the Colorado River, deposited in its delta on the gulf, and carried here by winds. In the adjoining Pinacate segment to the east, magma surging from the Earth's mantle blew the place up, repeatedly, leaving 400 cinder cones and ten massive craters to explore. At twilight, the reserve's biologist, Eugenio Larios, leads the way through labyrinths of jagged-edged lava. You fall, you bleed. Each step echoes below in tubular caves.

A mile ahead waits the entrance to one tunnel whose interior is the daytime maternity roost for about 150,000 lesser long-nosed bats. Leaving their young clinging to the walls, the mothers fly out each night to lap cactus nectar, sometimes venturing many miles into Arizona before dawn. They are nearing the end of a yearly migration from south to north through habitats with flowering cactuses—a nectar corridor from Central Mexico to southern Arizona. At dark, a torrent of winged mammals issues from the once molten ground. The cavern's breath drawn out in their wake is rank but soft with moisture. Bats rely on such humidity to temper Sonoran summer conditions, just as other mammals seek the refuge of damp burrows by day. After sitting a while, I lay one leg flat, and something strikes my calf like a hot needle. My headlamp beam spotlights a scorpion—a small, slender one. They're supposed to be the worst. We're a world away from a hospital. I ask Eugenio what to expect. He tells me that some people come out OK. And the others? He mimics keeling over, then shrugs.

Within minutes, I feel my heart beating too fast and a cold sweat on my skin. So I lie on the stone, elevate my foot, and wait for whatever comes next. Moonrise illuminates a distant mountain range to the south. A pocket mouse appears beside me, gathering dried grass seeds. Around a boulder comes a pinacate beetle, dark as the lava. When threatened, this insect lowers its head to point its abdomen skyward, ready to discharge foul-smelling, oily defensive chemicals. The indigenous Tohono O'odham say the pinacate beetle puts its head down out of shame because when its ancestor was given a load of stars to arrange in the sky, it tripped and spilled them. Above me is the result: the Milky Way, contending with the moon for brilliance. In an infinite universe stuffed with marvels, my own significance hardly seems worth !retting over. And after a while, it's apparent that the venom isn't affecting me anyway. My symptoms were from a flush of fear. I feel more alive than ever. Couch's spadefoot tadpoles have been growing in the water trapped by a small rock pit near a roadside.

Two days ago, they sprouted arms and webbed feet. Newly minted mini-adults are basking on the rim this morning. My afternoon trek up the arroyo is rewarding as well. Three pools persist, and so do clusters of tadpoles within them…. The Sonoran pronghorn never asked for much water. Unlike northern pronghorn, this desert subspecies gets almost all the moisture it needs in dry times from vegetation such as the pulpy ends of chain fruit chollas. Still, the population had been on the decline for decades. Sonora, Mexico, supported about 500. Another 150 lived north of the border, mainly in Arizona's 330,000-acre (134,000-hectare) Organ Pipe Cactus National Monument and 860,000-acre (348,000-hectare) Cabeza Prieta National Wildlife Refuge immediately to the west. Then, after several thirsty years in a row, the rains of 2001-02 failed almost completely. By late summer of 2002, the U.S.

pronghorn population totaled 19, vying with Florida panthers and mountain caribou for (he role of the nation s most endangered large mammal. Historically, these desert antelope could always retreat during severe drought to the Colorado River system or east to the Santa Cruz River and its tributaries. People forget that 19th-century trappers caught beavers and otters in streams shaded by ash, oak, cottonwood, and sycamore where only hot sand runs today. Ever increasing numbers of wells sunk into underlying aquifers have dramatically lowered water tables in the region. Excessive livestock grazing stripped away moisture-retaining riparian, or streamside, vegetation. So much surface water is siphoned off upstream for urban and agricultural use that the mighty Colorado's discharge into the GuIf of California has shrunk to dribbles of chemical-laden soup. And all the while, fences and other barriers, especially highways, discourage the shy pronghorn from migrating in search of what water and forage remain. "We had a sample of nine pronghorn wearing radio collars in 2001 and 2002," says Curt McCasland, Cabeza Prieta's assistant refuge manager.

"Nearly every one died. One female spent the last few days of her life right next to Highway 85"—a busy route through Organ Pipe Monument. "It had rained on the other side, but she wouldn't cross." If McCasland, refuge biologist Michael Coffeen, and others hadn't kept packing coolers of water into the wilderness for the staggering animals to find, perhaps none would have survived. The aid efforts didn't stop there. With support from private wildlife organizations, staff from the refuge and Arizona Game & Fish Department fenced a square mile of Cabeza Prieta range, added water, and brought in pronghorns from both Mexico and Arizona to breed in captivity. Today, the U.S. population is back up to 60 plus, including 14 adults and their offspring in an enclosure. In the first light of day, Allen Zufelt of the game department climbs a nearby hill and locates the animals with a telescope, focusing on a doe with twin fawns, the buck that fathered the young, and other captives until he is satisfied that all are healthy.

The next step in his daily routine is to walk the four-mile (six-kilometer) perimeter to be sure no openings have appeared in the woven wire fence or the electric fence in place around that. A double layer of security was called for to keep out coyotes, bobcats, and mountain lions and discourage illegal border-crossers from breaking through. "We get thousands of undocumented aliens a year coming through the refuge and Organ Pipe," Zufelt tells me. The people smugglers, labeled coyotes, and the drug smugglers, or narcotraficantes, have built up networks with lookouts, satellite phones, night-vision goggles, and modified high-speed vehicles that have cut tracks all through previously remote terrain. Some days, my naturalist treks have led me to more improvised shelters and abandoned gear—not to mention Border Patrol squads and Black Hawk helicopters—than wildlife. It's like being in the midst of a guerrilla conflict. Watching smoke rise from wildfires to the north, Zufelt calls it a sign of what some consider the most serious yet overlooked modern threat to the ecosystem: hardy grasses and weedy herbs from the Sahara and deserts in the Mediterranean and Asia.

A few, like buffel grass, were introduced as livestock forage. Forming a ground cover dense enough to fuel roaring blazes in an ecosystem never subject to wildfire before, the invaders could eventually replace slower growing, woody plants—in other words, the very cactuses and small trees that define the Sonoran Desert. Pronghorn mothers and fawns have joined in a nursery band just across the fence. The does move away in an effortless trot when they catch our scent. We catch theirs as the white rump hairs puff out in warning, releasing an almondy aroma from special glands. Zufelt points to a Harris's hawk nest with young atop a saguaro. "A pair of foxes dug into the pen," he recalls. "HaIf a dozen pronghorn bunched up and started chasing the foxes. One fox busy looking over its shoulder at a doe ran near this saguaro and was whacked on the side by an adult hawk defending the nest. That fox must have been thinking, Man, I've got to get outta here." Rain has once again refreshed the arroyo pools. Except for a Gila woodpecker in flight, nothing stirs on the sun-stunned mountainsides above.

Yet rain did fall, and more is predicted. Mirages dance over the valley, making it seem that the green plants shimmering below are merely an illusion. And yet rain fell. It always will, sooner or later. And thousands of years of wild invention will come into play to extract the most from every drop. North American Deserts - DesertUSA North America Deserts and World Deserts Hot Deserts - Cold Deserts In most modern classifications, the deserts of the United States and northern Mexico are grouped into four distinct categories. These distinctions are made on the basis of floristic composition and distribution -- the species of plants growing in a particular desert region. Plant communities, in turn, are determined by the geologic history of a region, the soil and mineral conditions, the elevation and the patterns of precipitation. Three of these deserts -- the Chihuahuan, the Sonoran and the Mojave -- are called "hot deserts," because of their high temperatures during the long summer and because the evolutionary affinities of their plant life are largely with the subtropical plant communities to the south. The Great Basin Desert is called a "cold desert" because it is generally cooler and its dominant plant life is not subtropical in origin.

Chihuahuan Desert : A small area of southeastern New Mexico and extreme western Texas, extending south into a vast area of Mexico. Great Basin Desert : The northern three-quarters of Nevada, western and southern Utah, to the southern third of Idaho and the southeastern corner of Oregon. According to some, it also includes small portions of western Colorado and southwestern Wyoming. Bordered on the south by the Mojave and Sonoran Deserts. Mojave Desert : A portion of southern Nevada, extreme southwestern Utah and of eastern California, north of the Sonoran Desert. Sonoran Desert : An arid region covering approximately 100,000 square miles in southwestern Arizona and southeastern California, as well as most of Baja California and the western half of the state of Sonora, Mexico. Subdivisions of this hot, dry region include the Colorado and Yuma deserts. Desert Disagreements This classification of North American deserts is by no means universally accepted by all biologists, geologists and other scientists. For instance, some maintain that the Mojave is not a distinct desert at all, but simply a transition zone between the Great Basin and Sonoran deserts.

Even among those who agree upon this classification, there is disagreement over the exact geographic areas circumscribed by each of the four deserts. Some scientists would use animals and other criteria, as well as distribution of plant species, to determine desert different boundaries for these four deserts. The Colorado Plateau is another major source of disagreement among scientists. This semiarid region of southern Utah and northern Arizona contains many majestic national parks, including Arches and Grand Canyon. Yet, experts cannot agree on the desert classification of this geologically distinct region. Some do not include the Colorado Plateau within any desert at all; others call this area the Painted Desert; still others, DesertUSA included, consider this region the southeastern extent of the Great Basin Desert. Grand Canyon from North Rim An additional source of confusion is provided by the great number of desert names (i.e. Yuman Desert, Colorado Desert, etc.) that do not match any of the four major deserts listed above. These names usually refer to local subdivisions within one of the four major deserts. These "deserts" are identified and defined below.

North American Desert Names & Subdivisions Arizona Upland Desert: That elevated portion of the Sonoran Desert in southern Arizona characterized by Saguaro Cactus. Black Rock Desert: A subdivision of the Great Basin Desert located in northwestern Nevada just northeast of Pyramid Lake. Borrego Desert: The term Borrego Desert is an informal term, rather than an official one. Officially there is no Borrego Desert. The term Borrego Desert as it is used informally refers to the northern portion (north of State Highway 78) of Anza-Borrego Desert State Park. Colorado Desert: The California portion of the Sonoran Desert west of the Colorado River. Anza-Borrego Desert State Park exists within the Colorado Desert. Escalante Desert: A subdivision of the Great Basin desert just west of Cedar Breaks in southwestern Utah. Great American Desert: An ill-defined, semiarid region of the Great Plains, or, all of the North American deserts combined. Great Sandy Desert: A subdivision of the Great Basin Desert located in southeastern Oregon . Northern Mojave Desert: The Mojave north of Las Vegas , Nevada. Magdalena Desert: The Sonoran Desert on the lowest third of the Baja Peninsula.

North American Desert: The vast arid region between the Rocky Mountains and Sierra Nevada of western North America, encompassing all four major American deserts. Oregon’s high desert country: Newberry National Volcanic Monument Painted Desert: This term is used differently by different writers: A narrow desert strip running west of the Grand Canyon, north-to-south along U.S. Route 89, then turning east along Interstate 40 to just beyond Petrified Forest National Monument . OR The entire region from the northern boundary of the Sonoran Desert of Arizona to southwestern Colorado and southern Utah, encompassing the Colorado River, the Colorado Plateau and its numerous parks and monuments. Red Desert: The semiarid region of southwestern Wyoming, sometimes considered an extension of the Great Basin Desert. Salton Trough is defined by the US Geological Survey as a a basin existing east of the Peninsular Ranges of Southern California and west of the San Andreas Fault in the vicinity of the Salton Sea. The Transverse Range at San Gorgonio Pass bounds it on the north. The southern end is in the Gulf of California. It’s considered a landward extension of the Gulf of California tectonic zone.

The Salton trough extends all the way to the Peninsular Range on the west, and Anza-Borrego Desert State Park is in it as well as the Coachella and Imperial Valleys. Sevier Desert: A subdivision of the Great Basin desert just northwest of Delta in south-central Utah. Smoke Creek Desert: A subdivision of the Great Basin Desert located in northwestern Nevada abutting the north end of Pyramid Lake. Southern Mojave Desert: The Mojave south of Las Vegas, Nevada. Trans-Pecos Desert: The Chihuahuan Desert west of Texas' Pecos River. Upland Desert: (See Arizona Upland Desert.) Vizcaino Desert: The Sonoran Desert on the middle third of the Baja Peninsula. Yuha Desert: That portion of the Sonoran Desert between Ocotillo and El Centro, California, south across the Mexican border. Yuma Desert: That portion of the Sonoran Desert just east of the Colorado River near Yuma, Arizona .

Deserts of the World The Sahara Desert: Geology and Climate - One of the most storied and unforgiving lands in the world, the Sahara -- the Arabic word for "desert" -- evokes a poignant sense of time and nature's power, of antiquity and legend, of wonder and mystery. Deserts of the World Deserts of the World The table below compares the sizes of the world's largest deserts. See Types of Deserts for more information about individual deserts throughout the world. North Central Mexico and Southwestern United States (Arizona, New Mexico, Texas) 175,000 mi2 High plateau covered by stony areas and sandy soil. Many mountains and mesas. cacti, chihuahuan flax, creosote bush, lechuguilla, mesquite, mexican gold poppy coyote, diamondback rattlesnake, javelina, kangaroo rat, roadrunner Largest North American desert. Big Bend National Park located here; more species of birds seen in Big Bend than in any other National Park in the U.S.

Great Basin Western United States (Idaho, Nevada, Oregon, and Utah) 158,000 mi2 411,000 km2 Covered by sand, gravel, and clay. Many moutains ranges, basins, and large expanses of salt flats. greasewood, sagebrush, shadscale bighorn sheep, jackrabbit, pocket mouse, poor-will, pronghorn antelope, sage thrasher, side-blotched lizard Great Salt Lake located here. Mojave Southwestern United States (Arizona, California, Nevada) 25,000 mi2 65,000 km2 Covered by sandy soil, gravelly pavement, and salt flats. creosote bush, desert sand verbena, joshua tree, mesquite bighorn sheep, chuckwalla, coyote, jackrabbit, sidewinder, zebra-tailed lizard Death Valley located in this desert. Sonoran Southwestern United States (Arizona, California) and parts of Mexico (Baja Peninsula, Sonora) 120,000 mi2 312,000 km2 Covered by sand, soil, and gravelly pavement.

Gets more rain than any other North American desert. agave, coulter's globemallow, creosote bush, desert mariposa lily, mesquite, ocotillo, paloverde, saguaro coati, elf owl, gila monster, kangaroo rat, pack rat, roadrunner, sidewinder, tarantula Most complex animal-plant community of any desert. Largest Desert in the World - Desert Map Home » Records » Largest Desert The World's Largest Deserts A map showing the generalized location of Earth's ten largest deserts and a table of over 20 major deserts. World desert map: This map shows the generalized location of Earth's ten largest deserts on the basis of surface area. The table at the bottom of this page provides the names, generalized locations, and surface areas of over twenty major deserts. Base map by NOAA. Sand dunes in the Sahara Desert of Libya: Most people think of deserts as "sandy" landscapes. That is true part of the time. This is a view of sand dunes in the Sahara Desert of Libya - an area known as the Ubari (or Awbari) Sand Sea. Photo © iStockphoto / PatrickPoendl.

What is a Desert? A desert is a landscape or region that receives very little precipitation - less than 250 mm per year (about ten inches). Approximately 1/3 of Earth's land surface is a desert. There are four different types of deserts based upon their geographic situation: 1) polar deserts, 2) subtropical deserts, 3) cold winter deserts, and 4) cool coastal deserts. As shown on the map above, deserts occur on all of Earth's continents. The Largest Desert The two largest deserts on Earth are in the polar areas. The Antarctic Polar Desert covers the continent of Antarctica and has a size of about 5.5 million square miles. The second-largest desert is the Arctic Polar Desert. It extends over parts of Alaska, Canada, Greenland, Iceland, Norway, Sweden, Finland, and Russia. It has a surface area of about 5.4 million square miles. McMurdo Dry Valleys: The largest deserts on Earth are in the polar regions. This is one of the McMurdo "dry valleys" near Lake Hoare, Antarctica. The Canada Glacier is in the background. Photograph by Peter West, National Science Foundation.

Non-Polar Deserts The rest of Earth's deserts are outside of the polar areas. The largest is the Sahara Desert, a subtropical desert in northern Africa. It covers a surface area of about 3.5 million square miles. A list of more than twenty of the largest non-polar deserts can be found below. Vegetation of the Sonoran Desert in Arizona: Cacti and grasses in Arizona's Sonoran Desert. Photo © iStockphoto / vlynder. The Desert Environment When most people think of a desert, they imagine a landscape covered with sand and sand dunes. Although many deserts are sand-covered, most are not. Many desert landscapes are rocky surfaces. They are rocky because any sand-size or smaller particles on the surface are quickly blown away. Rocky deserts are barren wind-swept landscapes. Most deserts receive so little precipitation that surface streams usually only flow immediately after rainfall - unless the stream has a source of water outside of the desert. Streams that enter a desert usually suffer major water losses before they exit. Some of the water is lost to evaporation. Some is lost to transpiration (taken up by plants and then released to the atmosphere from the plants).

And, some is lost to infiltration (water soaking into the ground through the bottom of the stream channel). Desert Fauna and Flora The plants and animals that live in a desert must be adapted to the environment. Plants must be very tolerant to intense sun, prolonged periods without precipitation, and have an ability to prevent moisture loss to conditions of severe temperature ranges, dry winds, and low humidity. Animals must be able to tolerate temperature extremes, temperature ranges, and have an ability to survive with very little water. Many animals adapt to desert conditions by living underground and being active at night. Major Deserts of the World Name

Question: Which is North America's largest desert?
Answer: Sonoran